The growing population of vehicles in the U.S. with Onboard Refueling Vapor Recovery (ORVR) equipment installed is resulting in increased vaporization occurring in underground storage tanks (USTs). When a vehicle equipped with an ORVR system refuels at a Gasoline Dispensing Facility (GDF), fresh air is returned to the tank instead of saturated vapor as in the case when a standard vehicle refuels. This returned fresh air causes increased evaporation of the fuel in the USTs. This increased evaporation results in continuously rising pressures in the ullage spaces of USTs. If pressures rise above ambient atmospheric pressure, fuel vapor emissions can occur because UST systems are not perfectly tight.
New Stage II Vapor Recovery requirements in the state of California require that UST systems maintain ullage pressures that average less than ¼ inch of water column above ambient pressure. Current front-end ORVR detection systems for GDFs provide active pressure control when the GDF is in operation and there are ORVR vehicles refueling, but may allow the UST to over pressurize during periods of station closure or low vehicle activity. To avoid the possibility of over pressurization of the UST, a back-end vapor processor is needed to process the excess vapor caused by evaporation.
Processors on the market today were designed to work on systems that do not use front-end ORVR detection systems to limit the amount of air that is ingested into the UST, and are expensive and overkill for this application. There is a need for a low cost solution capable of processing the small amount of vapor necessary to keep a UST system in compliance during periods of station shutdown or low activity. A prior solution that handled small amounts of vapor is the Healy Systems' bladder tank described in U.S. Pat. No. 6,805,173 B2. This solution is expensive and consists of a very large above ground tank with a flexible bladder liner. The large tank is difficult to locate and install at GDFs, and is not generally accepted by users but is required by current law in California. Users would prefer a cheaper, more palatable solution.